1. Field of the Invention
The present invention relates to a method and means for operating logarithmic circuits and, more particularly, to a method and apparatus for calibrating logarithmic circuits by storing known absorbance signals and subsequently determining the value of an unknown absorbance signal by comparison with the stored signals.
2. Description of the Prior Art
A variety of different types of instruments use log converter circuits for providing an output which is the log of the input or the reciprocal of the input. A spectrophotometer is an example of such an instrument. In a spectrophotometer, a beam of light of a known frequency is transmitted through a sample and a photomultiplier tube is positioned to detect the light passing through the sample. The less light absorbed by the sample, the more light is transmitted and the output of the photomultiplier tube is a current signal proportional to the intensity of the received light. Thus, the output of the photomultiplier tube is proportional to transmittance.
An operational amplifier is typically used to convert this current signal into a voltage signal and many circuits operate directly on this voltage signal, where transmittance is the desired output. On the other hand, spectrophotometers usually present sample light absorbance/transmittance data in units of absorbance (A) defined generally as: EQU A=log (1/T) (1)
where T is the fraction of light transmitted through the sample under test, with respect to a blank or standard sample.
Sophisticated instruments must concern themselves with two problems when using a photomultiplier tube and a log converter circuit, such as in a spectrophotometer. That is, it is desired that the relationship between absorbance and transmittance be exactly logarithmic and that a 100% transmittance signal correspond exactly to 0.0 absorbance signal. However, in practice, the output of a log converter circuit is rarely exactly 0.0A when the output of the photomultiplier tube is 100%T and it is difficult and expensive to construct highly accurate log converters which will provide an exactly logarithmic relationship between absorbance and transmittance.
In the past, it has generally been necessary to use expensive log converters and it is still usually necessary to require numerous electronic adjustments to tailor the output in a logarithmic fashion. The overall cost of a sophisticated instrument such as a spectrophotometer could be significantly reduced if there was a low cost, minimal calibration, analog log converter available. However, log converters meeting this requirement and still performing in a satisfactory manner have been unavailable heretofore.